While consumption of fossil fuel increases carbon dioxide in the air to remarkably cause deterioration of global environment, a solar cell is a semipermanent environment-friendly energy source since it uses solar light. For this reason, development of various types of solar cells has been attempted as development of future important energy sources. A solar cell is commonly employed in a form of a solar cell module wherein a solar cell element is sealed by an EVA (ethylene-vinyl acetate copolymer) and sandwiched between a transparent glass substrate and a back sheet (rear surface laminate) covering its front surface and rear surface, respectively.
The back sheet is provided to protect the EVA and the solar cell element, and a film to be employed as its outermost layer has to have a sufficient electric insulation property (volume resistivity: at least 1×1013 Ω·cm) and a sufficient moisture-proof property (water vapor permeation is at most 1 g/m2·24 h at 40° C. with a relative humidity of 90%).
Further, since a solar cell module is exposed to outdoor environment for a long time, the back sheet has to have a sufficient durability.
Specifically, since exposure to a carbon arc type sunshine weather meter (SWM) for from 250 to 500 hours corresponds to one year of exposure to outdoor environment, it has been necessary to suppress strength reduction by 5,000 hour exposure to SWM (corresponding to from 10 to 20 years) to be at most a half of the initial strength. Further, since the actual temperature of a solar cell is at most about 90° C., the solar cell has been required to have a thermal resistance sufficient for suppressing strength reduction under 90° C., 20 year use to be a degree causing no practical problem.
As a film to be employed as the outermost layer of the back sheet, a fluororesin film employing a fluororesin such as ETFE (ethylene-tetrafluoroethylene copolymer), PVF (polyvinyl fluoride) and PVDF (polyvinylidene fluoride), that have the above properties, have been known. Among these, an ETFE film or a PVDF film undergoes no reduction of strength due to hydrolysis even under the condition of 85° C., a relative humidity of 85% for 1,000 hours. Further, with an ETFE film, the temperature at which the elongation degree lowers to a half by a thermal resistance test of 100,000 hours (about 10 years), is about from 150 to 160° C.
As a back sheet employing such a fluororesin film as the outermost layer, a white film is mainly employed, but from the viewpoint of improving a design, a fluororesin film containing a black pigment such as a carbon black has also been shown (for example, refer to Patent Documents 1 and 2). Further, in an amorphous type solar cell, since deterioration of its solar cell element due to light can be suppressed by maintaining the temperature of the solar cell to be high to a certain extent, an embodiment of containing a black pigment is shown (refer to Patent Document 3).